1. Field of the Invention
The present invention relates to a radiation image readout apparatus for reading out, by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, a radiation image stored on a stimulable phosphor sheet or the like.
2. Description of the Related Art
There are known stimulable phosphors, which upon the irradiation thereof by radiation store a portion of the radiation energy, and emit upon the subsequent irradiation thereof with a visible or other excitation light a stimulated emission corresponding to the stored radiation energy. Radiation image readout apparatuses employing, for example, stimulable phosphor sheets, which utilize these stimulable phosphors in a layer containing stimulable phosphors which has been formed on a substrate, to temporarily record the radiation image data of the radiation that has passed through a human body or other subject of photographing, upon the irradiation thereof by a radiation, are in wide use as CR (Computed Radiography) apparatuses.
An image signal obtained by the system described above is subjected to image processing such as gradation processing, frequency processing, etc. appropriate to diagnostic reading, after which the processed image signal is recorded on a film as a visible diagnostic image (final image), or displayed on a high-resolution CRT monitor for diagnostic reading by a doctor.
Further, in accordance with the radiation image readout apparatus utilizing the stimulable phosphor sheet described above and with a view to making the apparatus compact and of reduced cost, a line light source is used as an excitation light source for irradiating the sheet with an excitation light in a line form, together with a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line along the lengthwise direction of the line-shaped portions of a stimulable phosphor sheet that has been irradiated with an excitation light emitted from the line light source (hereinafter referred to as the main scanning direction); a scanning means for moving the line light source and line sensor from one end of the stimulable phosphor sheet to the other end in the direction substantially perpendicular to the lengthwise direction of the line-shaped portions (hereinafter referred to as the sub-scanning direction) (refer to Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540).
However, when a stimulated emission based on a radiation image is received by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, because the photoelectric converting elements are arranged at a uniform pitch, the stimulated emission is sampled at every pixel; accordingly, at this time, for cases in which the signal component of the stimulated emission received by the line sensor, based on a radiation image scanned by the line light source, includes a signal component having a frequency higher than xc2xd the sampling frequency (hereinafter referred to as the Nyquist frequency), a Moirxc3xa9 effect is caused in the final image, whereby the image quality thereof is deteriorated.
The radiation image readout apparatus according to the present invention has been developed in consideration of the circumstances described above, and it is a primary objective of the present invention to provide a radiation image readout apparatus for reading out, by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, a radiation image stored on a stimulable phosphor sheet or the like, wherein the occurrence of a Moirxc3xa9 effect in the final image is controlled by preventing distortion due to feedback.
The first radiation image readout apparatus according to the present invention comprises: a illuminating means for illuminating with an excitation light in a line form a portion of the surface of a stimulable phosphor sheet on which a radiation image has been stored; a focusing optical system provided with a focusing lens for focusing the stimulated emission emitted from a portion of the stimulable phosphor sheet upon the illumination thereof by the excitation light emitted from the illuminating means in a line form and a detecting means provided with a line sensor, which is formed of a plurality of photoelectric converting elements arranged in a straight line and at a uniform pitch, for receiving and photoelectrically converting a stimulated emission focused by the focusing optical system; a scanning means for moving the illuminating means and the detecting means relatively from one end of the stimulable phosphor sheet to another in a direction differing from that of the lengthwise direction of the illuminated portions, wherein the MTF of the focusing optical system is 50% or less of the Nyquist frequency determined by the aforementioned uniform pitch.
Here, the referent of xe2x80x9cthe focusing lens of the focusing optical systemxe2x80x9d is not limited to being a single lens, but includes systems provided with a plurality of focusing lenses, as well as systems provided with elements for performing an optical function other than focusing.
Further, the referent of xe2x80x9cthe MTF of the focusing optical systemxe2x80x9d as shown in FIG. 4, for example, falls in inverse proportion to the rising of the frequency of the signal component of a stimulated emission based on a radiation image. Further, the phrase xe2x80x9cthe Nyquist frequency determined by the aforementioned uniform pitchxe2x80x9d refers to the frequency equivalent to xc2xd the sampling frequency, which is determined by the pitch at which the photoelectric converting elements of the line sensor are disposed. Generally, if there is a signal component having a frequency higher than the Nyquist frequency, distortion due to feedback is produced. The expression xe2x80x9cthe MTF of the focusing optical system is 50% or less of the Nyquist frequency determined by the aforementioned uniform pitchxe2x80x9d means that the signal component occurring in the Nyquist frequency is caused to become 50% or less by the MTF of the focusing optical system.
Still further, the MTF of the focusing optical system can also be caused to be 20% or less of the Nyquist frequency determined by the aforementioned uniform pitch, as shown in FIG. 5.
The second radiation image readout apparatus according to the present invention comprises: a illuminating means for illuminating with an excitation light in a line form a portion of the surface of a stimulable phosphor sheet on which a radiation image has been stored; a focusing optical system provided with a focusing lens for focusing the stimulated emission emitted from a portion of the stimulable phosphor sheet upon the illumination thereof by the excitation light emitted from the illuminating means in a line form; a detecting means provided with a line sensor, which is formed of a plurality of photoelectric converting elements arranged in a straight line and at a uniform pitch, for receiving and photoelectrically converting a stimulated emission focused by the focusing optical system; a scanning means for moving the illuminating means and the detecting means relatively from one end of the stimulable phosphor sheet to another in a direction differing from that of the lengthwise direction of the illuminated portions; and a readout means for reading out the output of the line sensor in the order corresponding to the movement thereof, and obtaining the data forming the final image; wherein the aforementioned uniform pitch is less than the width of the pixels of the final image; further comprising a pixel-density converting filter for converting the image signal of the pixel density output from the line sensor based on the aforementioned uniform pitch to the pixel density of the final image; wherein the frequency characteristic of the pixel-density converting filter is caused to be 50% or less of the Nyquist frequency determined by the pixel density of the final image.
Here, the expression xe2x80x9cthe aforementioned uniform pitch is less than the width of the pixels of the final imagexe2x80x9d refers to the raising of the sampling frequency by causing the aforementioned uniform pitch to be narrower than the width occurring in the main scanning direction of the pixels of the final image so as to reduce the distortion due to feedback. Further, the referent of xe2x80x9cthe frequency characteristic of the pixel-density converting filterxe2x80x9d is, for example, the characteristic whereby the frequency range converted by the pixel-density converting filter falls in inverse proportion to the rising of the frequency of the signal component of a stimulated emission based on the radiation image shown in FIG. 8, for example. Further, the expression xe2x80x9cthe Nyquist frequency determined by the pixel density of the final imagexe2x80x9d is the frequency that is xc2xd of the sampling frequency determined by the width occurring in the main scanning direction of the pixels of the final image. The expression xe2x80x9cthe frequency characteristic of the pixel-density converting filter is caused to be 50% or less of the Nyquist frequency determined by the pixel density of the final imagexe2x80x9d refers to the causing, according to the frequency characteristic of the pixel-density converting filter, of the signal component of the stimulated emission occurring in the Nyquist frequency to be 50% or less when the converting of the pixel density occurring in the main scanning direction is performed thereby.
Note that because the frequency characteristic of the pixel-density converting filter is determined by the filtering coefficient and the number of filter taps thereof, a desired frequency characteristic can be realized by selecting an appropriate filtering coefficient and number of filter taps.
Further, the frequency characteristic of the pixel-density converting filter can also be caused to be 20% or less of the Nyquist frequency determined by the pixel density of the final image, as shown in FIG. 9.
Still further, according to the first and second radiation image readout apparatuses of the present invention: the width of the interval in which the stimulable phosphor sheet is relatively moved in the direction relative to the portions that have been illuminated by the excitation light in a line form, is narrower than the width of the pixels of the final image; a second pixel-density converting filter is provided for converting to the pixel density of the final image the image signal of the pixel density based on the width of the interval in the direction in which the illuminated portions outputted by the line sensor are relatively moved; and the frequency characteristic of the second pixel-density converting filter can be caused to be 50% or less of the Nyquist frequency determined by the width of the pixels of the final image.
Here, the expression xe2x80x9cthe width of the interval in which the stimulable phosphor sheet is moved in the direction relative to the portions that have been illuminated by the excitation light in a line form, is narrower than the width of the pixels of the final imagexe2x80x9d means that, as shown in FIG. 10, the width of the sub-scanning direction (the direction Y) of the portions of the stimulable phosphor sheet that have been illuminated by the excitation light in a line form is smaller than the width of one pixel (in FIG. 10, xc2xc the width of a pixel).
Accordingly, the expression xe2x80x9cfor converting to the pixel density of the final image the image signal of the pixel density based on the width of the interval in the direction in which the illuminated portions outputted by the line sensor are relatively movedxe2x80x9d means that the image signal of one pixel of the final image is converted so that the sub-scanning direction (the direction Y) of the portions of the sheet illuminated by the excitation light is configured of a plurality of image signal areas into which the width thereof has been divided.
Further, the expression xe2x80x9cthe Nyquist frequency determined by the pixel density of the final imagexe2x80x9d refers to the frequency that is equivalent to xc2xd the scanning frequency determined by the width of the pixels occurring in the sub-scanning direction of the final image. Further, the expression xe2x80x9cthe frequency characteristic of the of the second pixel-density converting filter can be caused to be 50% or less of the Nyquist frequency determined by the width of the pixels of the final imagexe2x80x9d refers to the causing of the signal component occurring in the Nyquist frequency to be 50% or less due to said frequency characteristic when the pixel density occurring in the sub-scanning direction is converted by the pixel-density converting filter of the second radiation image readout apparatus according to the present invention.
Further still, the frequency characteristic of the second pixel-density converting filter can be caused to be 20% or less of the Nyquist frequency determined by the pixel density of the final image, as shown in FIG. 9, for example.
According to the first radiation image readout apparatus of the present invention, because the MTF of the focusing optical system for focusing the stimulated emission, which is based on a radiation image, emitted from a stimulable phosphor sheet upon the illumination thereof by an excitation light is caused to be less than 50% of the Nyquist frequency determined by the uniform pitch at which the photoelectric converting elements of the line sensor are disposed, the distortion due to feedback can be reduced, and the occurrence of a Moirxc3xa9 effect in the final image can thereby be controlled.
Further, for cases in which the MTF of the focusing optical system has been caused to be 20% or less of the Nyquist frequency determined by the aforementioned uniform pitch, the distortion due to feedback can be further reduced, and the occurrence of a Moirxc3xa9 effect in the final image can thereby be controlled.
According to the second radiation image readout apparatus of the present invention: the width of the pitch of the photoelectric converting elements of the line sensor for receiving the stimulated emission emitted from the stimulable phosphor sheet upon the illumination thereof with the excitation light is made to be narrower than the width of the pixels of the final image, whereby the Nyquist frequency is raised; a pixel-density converting filter is provided for converting the pixel density of the image signal obtained by the reception and photoelectrical conversion by the line sensor of the stimulated emission to the desired pixel density of the final image; and because the frequency characteristic of the of the pixel-density converting filter has been caused to be 50% or less of the Nyquist frequency, which is determined by the pixel density of the pixels of the final image, the distortion due to feedback can be reduced, and the occurrence of a Moirxc3xa9 effect in the final image can thereby be controlled.
Further, for cases in which the frequency characteristic of the of the pixel-density converting filter has been caused to be 20% or less of the Nyquist frequency determined by the aforementioned uniform pitch, the distortion due to feedback can be further reduced, and the occurrence of a Moirxc3xa9 effect in the final image can thereby be controlled.
According to the first and second radiation image readout apparatuses of the present invention: the width of the interval in the direction in which a portion of the stimulable phosphor sheet that has been illuminated with the excitation light in a line form is relatively moved is narrower than the width of the pixels of the final image; a second pixel-density converting filter is provided for converting the pixel density based on the width of the direction in which the image signal output by the line sensor is relatively moved to the pixel density of the final image; for cases in which the frequency characteristic of the of the second pixel-density converting filter has been caused to be 50% or less of the Nyquist frequency determined by the width of the pixels of the final image, distortion due to feedback occurring in the pixel-density conversion occurring in the sub-scanning direction can also be reduced, and the occurrence of a Moirxc3xa9 effect in the final image can be further controlled.
Further, for cases in which the frequency characteristic of the of the second pixel-density converting filter has been caused to be 20% or less of the Nyquist frequency determined by the width of the pixels of the final image, the occurrence of a Moirxc3xa9 effect in the final image can be more effectively controlled in the same manner as described above.